1. Field of the Invention
The present invention relates to an optical module, and more particularly, to a bi-directional optical module simultaneously transmitting and receiving optical signals having different wavelengths through a single optical fiber.
2. Description of the Related Art
In general, an individual subscriber transmits and receives an optical signal through a strand of optical fiber on a network such as an optical subscriber network. Such a single optical fiber distribution network can reduce cost for installing an optical fiber. Thus, a communication network can be built at a low cost. Two wavelengths are used to transmit and receive an optical signal. In general, a wavelength of 1.3 μm is used to transmit the optical signal, and a wavelength of 1.5 μm is used to receive the optical signal. A bi-directional optical module is used to simultaneously transmit and receive optical signals having different wavelengths using a single optical fiber.
FIG. 1 is a schematic cross-sectional view of a bi-directional optical module according to the prior art.
In detail, FIG. 1 is a schematic cross-sectional view of a bi-directional optical module disclosed on Jun. 30, 1992 in U.S. Pat. No. 5,127,075 invented by Althaus et al. Referring to FIG. 1, the bi-directional optical module includes a transistor outline (TO) type laser diode (LD) module 10, a TO type photo diode (PD) module 20, and a wavelength division multiplexing (WDM) filter 30. The TO type LD module 10 includes an LD 12, a monitor PD 14, a lens 16, and an LD cap 18. The monitor PD 14 is used to monitor light having a wavelength of 1.3 μm, the light being output from the LD 12.
The TO type PD module 20 includes a PD 22, a lens 24, and a PD cap 26. The TO type LD module 10 and the TO type PD module 20 are connected to each other through a common housing 34. The WDM filter 30 reflects light having a wavelength of 1.5 μm, the light being incident from an optical fiber 32, and being received in the TO type PD module 20 as marked with chain lines and transmits light having a wavelength 1.3 μm, the light being output from the TO type LD module 10, through a lens 28 and the optical fiber 32. In other words, the WDM filter 30 spatially splits the light having the wavelengths of 1.3 μm and 1.5 μm.
However, in the bi-directional optical module shown in FIG. 1, the common housing 34 must be highly precisely manufactured. Also, in the bi-directional optical module shown in FIG. 1, the TO type LD module 10 and the TO type PD module 20 must be separately manufactured and then attached to the common housing 34. Thus, manufacturing of the bi-directional optical module is costly.
Structures of a general TO type LD module and the TO type LD module 10 shown in FIG. 1 will now be described in more detail.
FIGS. 2 through 4 are front and side cross-sectional views and a plan view of a general TO type LD module.
In detail, a TO type LD module 50 shown in FIGS. 2 through 4 uses a TO type package. In a basic structure of the TO type package, a sub-mount pedestal 54 is vertically attached to a circular header base 52. The circular header base 52 encloses a cylindrical cap 64, and a window 66 is formed in a center of the cylindrical cap 64 so as to pass light. A lead pin 62 protruding outside is attached to the circular header base 52.
An LD 56 constituting the TO type LD module 50 is attached to a sub-mount 58 and then to a sub-mount pedestal 54, and a monitor PD 60 is attached to the circular header base 52 through the sub-mount 58. The LD 56 and the monitor PD 60 are electrically connected to the lead pin 62. The lead pin 62 is connected to the LD 56 and the monitor PD 60 through wires 68 using wire bonding.
The LD 56 outputs light toward a window 66 and the circular header base 52. The light being output toward the window 66 is combined with an optical fiber (not shown) through a lens, and the light being output toward the circular header base 52 is detected as a signal by the monitor PD 60 so as to monitor an output of the LD 56.
The TO type package adopted in the TO type LD modules 10 and 50 shown in FIGS. 1 through 4 remarkably succeeds in terms of industry and is used in a compact disc player, a laser pointer, a bar-code scanner, or the like. Such a TO type package can be produced at a considerably low cost, and light power coincides with a central axis of the circular header base 52 so as to be easily arranged with and fixed to an optical system such as an optical fiber or the like. The TO type package can be intercepted from the outside by the window 66 and a cap 64 so as to protect the LDs 12 and 56 that may be damaged by moisture or the like. The attachment of an optical fiber to the TO type package with good light coupling is an established process and thus can be achieved at a low cost.
A bi-directional optical module including a single TO type package has been suggested to use the advantages of such a TO type package. An example of such a bi-directional optical module includes U.S. Pat. No. 6,493,121, entitled “Bi-directional Module for Multi-channel Use” by Althaus et al. on Dec. 10, 2002. Also, such a bi-directional optical module has been described in “Compact Bi-directional Optical Module Using Ceramic Blocks” by H.-J. Yoon, “IEEE Photonics Technology Letters, Vol. 16, No. 8, 2004, pp. 1954-1956.” However, in such a bi-directional optical module, a TO type package requires many parts that need to be precisely processed. Thus, cost for manufacturing the bi-directional optical module is increased.
FIG. 5 is a schematic cross-sectional view of a bi-directional device according to the prior art.
In detail, FIG. 5 illustrates a disclosure, entitled “−31 dBm Sensitivity of a Monolithic Transmit-receive Device over Wide Temperature Range” by Mallecot et al. in “Optical Fiber Communication Conference 1999 Technical Digest, Vol. 3, pp. 191-194.” A bi-directional device 70 shown in FIG. 5 is manufactured by integrating an LD 72, an absorber 74 attenuating light output from the LD 72 toward a PD 76, and the PD 76 detecting light into a single chip. The LD 72 includes an active layer 82 generating light having a wavelength of 1.3 μm inside a substrate 78, and the PD 76 includes the active layer 82 detecting light having a wavelength of 1.5 μm inside the substrate 78. As shown in FIG. 5, reference numeral 84 denotes a grating constituting the LD 72, reference numeral 86 denotes an LD electrode, reference numeral 88 denotes an absorber electrode, reference numeral 90 denotes a PD electrode, reference numeral 94 denotes a common electrode, and reference numeral 92 denotes an electrical isolation layer.
However, sensitivity of −31 dBm was obtained at an optical transmission speed of 155 Mbit/s in the bi-directional device adopting the structure shown in FIG. 5, and PD responsivity was 0.3 A/W. In general, sensitivity of a receiver is expressed as in Equation 1:Sensitivity=10 log(SN·ln(re+1)·1000)/(2ρ·(re−1))dBm  (1)wherein SN denotes a signal-to-noise ratio, re denotes an extinction ratio, In denotes a noise current A of a transimpedance amplifier (TIA) connected to a PD, and ρ denotes module responsivity. In a case where the TIA is formed of a field effect transistor (FET), a noise current increases in the form of square root with an increase in a bandwidth. When an optical transmission speed is 1.25 Gbit/s, responsivity of the PD 76 must be 0.85 A/W to get the same sensitivity. It is very difficult to obtain high PD responsivity with the bi-directional device as shown in FIG. 5 because of reasons described in the following paragraph. Thus, high module receive sensitivity cannot be obtained.
In the bi-directional device shown in FIG. 5, an optical signal incoming to the PD 76 is coupled from an optical fiber (not shown) to the LD 72 and propagated through the LD 72, the optical signal having a wavelength of 1.5 μm. Optical coupling efficiency between the LD 72 and the optical fiber is lower than that between the typical surface incidence type PD and the optical fiber. Highly doping of impurities on a p-clad layer for a high temperature operation of the LD 72 increases loss of an optical signal caused by intervalence band absorption. An absorbing waveguide may be inserted between the LD 72 and PD 76 to prevent an optical crosstalk of the PD 76 caused by an optical signal output from the LD 72, the optical signal having a wavelength of 1.3 μm. The absorbing waveguide causes loss of an optical signal. Thus, the optical signal being input toward the PD 76, the optical signal having the wavelength of 1.5 μm, becomes weak due to the losses caused by the optical coupling and the waveguide, and thus responsivity of the PD 76 is lowered.
In addition, the bi-directional device shown in FIG. 5 has a single chip form into which an LD, an absorber, and a PD are integrated. Thus, the LD requires a forward voltage, while the PD requires a reverse voltage. As a result, two power supplies are required. Moreover, in the bi-directional device shown in FIG. 5, the LD, the absorber, and the PD are placed on a substrate. Thus, an electrical crosstalk between the LD and the PD is very large at a high transmission speed.